The general problem of digital signaling over fading multi-path channels is described by Proakis, Digital Communications, McGraw-Hill, Inc., 1983 in Chapter 7. In a very elementary sense, the multi-path channel is a model for describing why some communication channels fade in and out, various causes being possible, and not pertinent to the present discussion. When using error correction decoding, the problem is what to do when the channel is known to fade in and out sporadically.
One well-known solution is to use equalization, in which feedforward and feedback data provides a predictive correction to each sample of the received signal. In this technique, some of the data already received has been demodulated, providing a highly reliable estimate of the received signal, while the remaining portion has not yet been demodulated and is therefore not as reliable. The received data is viewed through a time window in which the current signal sample sits in the temporal middle, some of the previous data having been corrected, and is therefore available as "decision feedback" data. Each sample in the time window is multiplied by some equalizer coefficient. The received sample value is compared with the predictive correction to the received sample generated by the equalizer when it later becomes available, the difference then being employed to provide a better estimate for the value of each of the equalizer coefficients. Probably the best known example of such an algorithm is the Kalman algorithm described in the above referenced text by Proakis beginning at page 421.
The approach taken in the present invention is to combine error correction encoding/decoding with equalization. However, a straight-forward combination of the equalizer and an error correction decoder suffers from the disadvantage that the output of the error correction decoder (which is the most reliable feedback) is delayed by at least one codeword time, so that the comparison of the predictive correction generated by the equalizer with the corresponding decoded symbol generated by the decoder is not available until long after the equalizer has processed the corresponding signal sample. This either postpones the updating of the equalizer coefficients by the Kalman algorithm and therefore impedes overall performance, or necessitates using less reliable data that has not yet been decoded as feedback.
One technique for dealing with a fading channel is a subject of U.S. Pat. No. 4,559,625 by Elwyn R. Berlekamp and Po Tong, entitled "Interleavers for Digital Communications". In this technique, plural codewords are helically interleaved together to form a single stream of data. The receiver has a de-interleaving array corresponding to the interleaving array of the transmitter. The de-interleaving provides decoded versions of those symbols transmitted immediately prior to the currently received sample. In the presence of long error bursts, typical of fading channels, the current received signal is easily predicted as an erasure before it enters the decoder, since it will have been preceded immediately by a series of symbols which are known to have been decoded as erasures. Thus, the location of the current symbol is provided to the decoder as an erasure location before the decoder begins decoding the codeword including the current symbol, thus saving decoding time and increasing the decoder's apparent error correction capacity.
A variation on the aforementioned technique of U.S. Pat. No. 4,559,625 is the use of an equalizer (using the Kalman algorithm) in combination with the helical interleaving of the codewords. This variation employs previously decoded symbols as feedback data, and the Kalman algorithm is provided with a known constant value for .tau..sub.0, the channel decorrelation time. This variation was simulated in a demonstration pursuant to a contract with a customer by Cyclotomics, Inc. in 1984.